[merged mm-stable] memcg-mm_update_next_owner-kill-the-retry-logic.patch removed from -mm tree

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The quilt patch titled
     Subject: memcg: mm_update_next_owner: kill the "retry" logic
has been removed from the -mm tree.  Its filename was
     memcg-mm_update_next_owner-kill-the-retry-logic.patch

This patch was dropped because it was merged into the mm-stable branch
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Oleg Nesterov <oleg@xxxxxxxxxx>
Subject: memcg: mm_update_next_owner: kill the "retry" logic
Date: Wed, 26 Jun 2024 17:29:24 +0200

Add the new helper, try_to_set_owner(), which tries to update mm->owner
once we see c->mm == mm.  This way mm_update_next_owner() doesn't need to
restart the list_for_each_entry/for_each_process loops from the very
beginning if it races with exit/exec, it can just continue.

Unlike the current code, try_to_set_owner() re-checks tsk->mm == mm before
it drops tasklist_lock, so it doesn't need get/put_task_struct().

Link: https://lkml.kernel.org/r/20240626152924.GA17933@xxxxxxxxxx
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Jinliang Zheng <alexjlzheng@xxxxxxxxxxx>
Cc: Mateusz Guzik <mjguzik@xxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Tycho Andersen <tandersen@xxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 kernel/exit.c |   57 ++++++++++++++++++++++--------------------------
 1 file changed, 27 insertions(+), 30 deletions(-)

--- a/kernel/exit.c~memcg-mm_update_next_owner-kill-the-retry-logic
+++ a/kernel/exit.c
@@ -439,6 +439,23 @@ static void coredump_task_exit(struct ta
 }
 
 #ifdef CONFIG_MEMCG
+/* drops tasklist_lock if succeeds */
+static bool try_to_set_owner(struct task_struct *tsk, struct mm_struct *mm)
+{
+	bool ret = false;
+
+	task_lock(tsk);
+	if (likely(tsk->mm == mm)) {
+		/* tsk can't pass exit_mm/exec_mmap and exit */
+		read_unlock(&tasklist_lock);
+		WRITE_ONCE(mm->owner, tsk);
+		lru_gen_migrate_mm(mm);
+		ret = true;
+	}
+	task_unlock(tsk);
+	return ret;
+}
+
 /*
  * A task is exiting.   If it owned this mm, find a new owner for the mm.
  */
@@ -446,7 +463,6 @@ void mm_update_next_owner(struct mm_stru
 {
 	struct task_struct *c, *g, *p = current;
 
-retry:
 	/*
 	 * If the exiting or execing task is not the owner, it's
 	 * someone else's problem.
@@ -468,16 +484,16 @@ retry:
 	 * Search in the children
 	 */
 	list_for_each_entry(c, &p->children, sibling) {
-		if (c->mm == mm)
-			goto assign_new_owner;
+		if (c->mm == mm && try_to_set_owner(c, mm))
+			goto ret;
 	}
 
 	/*
 	 * Search in the siblings
 	 */
 	list_for_each_entry(c, &p->real_parent->children, sibling) {
-		if (c->mm == mm)
-			goto assign_new_owner;
+		if (c->mm == mm && try_to_set_owner(c, mm))
+			goto ret;
 	}
 
 	/*
@@ -489,9 +505,11 @@ retry:
 		if (g->flags & PF_KTHREAD)
 			continue;
 		for_each_thread(g, c) {
-			if (c->mm == mm)
-				goto assign_new_owner;
-			if (c->mm)
+			struct mm_struct *c_mm = READ_ONCE(c->mm);
+			if (c_mm == mm) {
+				if (try_to_set_owner(c, mm))
+					goto ret;
+			} else if (c_mm)
 				break;
 		}
 	}
@@ -502,30 +520,9 @@ retry:
 	 * ptrace or page migration (get_task_mm()).  Mark owner as NULL.
 	 */
 	WRITE_ONCE(mm->owner, NULL);
+ ret:
 	return;
 
-assign_new_owner:
-	BUG_ON(c == p);
-	get_task_struct(c);
-	/*
-	 * The task_lock protects c->mm from changing.
-	 * We always want mm->owner->mm == mm
-	 */
-	task_lock(c);
-	/*
-	 * Delay read_unlock() till we have the task_lock()
-	 * to ensure that c does not slip away underneath us
-	 */
-	read_unlock(&tasklist_lock);
-	if (c->mm != mm) {
-		task_unlock(c);
-		put_task_struct(c);
-		goto retry;
-	}
-	WRITE_ONCE(mm->owner, c);
-	lru_gen_migrate_mm(mm);
-	task_unlock(c);
-	put_task_struct(c);
 }
 #endif /* CONFIG_MEMCG */
 
_

Patches currently in -mm which might be from oleg@xxxxxxxxxx are

zap_pid_ns_processes-dont-send-sigkill-to-sub-threads.patch
coredump-simplify-zap_process.patch





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